Which American states allow medical marijuana usage?

Drugs and Medicines
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When marijuana is smoked, tetrahydrocannabinol (THC), its active ingredient, is absorbed primarily in the fat tissues. The body transforms the THC into metabolites, which can be detected by urine tests for up to a week. Tests involving radioactively labeled THC have traced the metabolites for up to a month. The retention of labeled THC in humans is about 40 percent at three days and 30 percent at one week.

Since 1996, 15 states and the District of Columbia have passed laws allowing the use of marijuana for medical purposes. These include Alaska, Arizona, California, Colorado, District of Columbia, Hawaii, Maine, Michigan, Montana, Nevada, New Jersey, New Mexico, Oregon, Rhode Island, Vermont, and Washington. Depending on the state, patients may posess between 1 ounce (Alaska, Montana, Nevada) to 24 ounces (Oregon, Washington) of usable marijuana and grow their own plants. States also vary on which diseases are approved for treatment using marijuana.